Polymer films formed from ultraviolet (UV)-curable ink compositions are used as planarization and decoupling layers in the multilayered thin film encapsulation of organic light-emitting diode (OLED) devices and other electronic devices, in order to prevent exposure of the devices to moisture and oxygen. The multilayer thin films used for the encapsulation of the devices are composed of stacks of alternating layers of the polymeric films and inorganic barrier layers.
A desirable feature of cured polymeric films used in thin film encapsulation is a high degree of cure, which provides the films with an environmental stability due to reduced free volume and, therefore, lower water diffusion and solubility in the film. Unfortunately, conventional ultraviolet curing processes use high-intensity ultraviolet radiation that can lead to fast vitrification, which limits the degree of curing.
Another desirable feature of the polymeric films used in thin film encapsulation is low outgassing. This is because volatile chemical species from the polymeric films that become trapped inside the encapsulation stacks can negatively impact the performance of the encapsulated device. By diffusing in the polymeric film and through pinholes of the inorganic barrier layer adjacent to a device, volatile species may react with active functional layers in the device, deteriorating its performance characteristics. In addition, mobile volatile species may accumulate at point defects in the device structure, producing bubbles. These bubbles may lead to delamination of layers inside the device itself (e.g., cathode delamination), producing black spots, or inside the multilayered encapsulation stack (e.g., between the inorganic barrier layers and the polymeric layers), deteriorating the optical properties and/or the integrity of the device.
Photoinitiators, which are used to start the monomer polymerization reaction, are a key component in UV-curable ink compositions. However, photoinitiator molecules and the by-products of their degradation reactions are a common source of volatile species in UV-cured polymeric films. Upon UV curing, particularly upon UV curing with monochromatic LED lamps in the A-UV range, a fraction of the photoinitiator in a UV-curable ink composition may remain unactivated by the UV-radiation. In addition, fragments of the activated photoinitiator molecules that do not become bound to the polymeric network can remain in the polymeric films as volatile species that are left free to migrate in the polymeric film and inside the multilayered encapsulation stack.